Historically, a stylus is an elongated, sharp, pointed instrument used for writing, marking, and engraving. More recently styli are being modified for use with inputting data to computer devices.
In the context of touch screen computer interfaces, a stylus provides many benefits to users. For example, a user can more accurately use a stylus for computer touch screen input than they can their own finger. The accuracy is due to the fact that a computer stylus has a smaller tip than do most human fingers, and so can achieve an accordingly higher degree of accuracy on a touch screen. This increase in accuracy, in turn, allows for smaller user interface elements, and provides increased ease of use for many users. Additionally, some users prefer to use a stylus simply to avoid depositing the natural oils from their hands on the screen.
One disadvantage to stylus use is that it necessitates carrying an additional personal item. This is especially problematic given the already large number of personal items commonly carried by individuals such as: keys, pens, glasses, wallet, and a smart phone. One solution to this problem is the combination pen and stylus.
A touch screen is, generally speaking, a combination touchpad and computer display that can detect the presence and location of a touch within the display area. Although this patent application will refer generally to touch screens, much of the technology disclosed herein will work with other similar human machine interfaces, such as a touchpad. There are many touch screen technologies including resistive, capacitive, surface acoustic wave, infrared, strain gauge, optical imaging, dispersive signal technology, inductive sensor systems that may be placed under an LCD, and acoustic pulse recognition. Of these, the first two (resistive and capacitive) are the most common.
Resistive touch screens have been used on smart phones and tablet computers. An example of a resistive touch screen is the PALM PILOT®. Resistive touch screens comprise two very slightly separated optically transparent sheets, at least one flexible, and both coated with a transparent electrically conductive material. Normally, there is no contact between the two sheets, however, when the surface of the touch screen is touched at a point by a stylus or other object, the two sheets contact each other at that point, registering to the related computer system the precise location of the touch. This type of touch screen can sense contact from nearly any solid object pressed against it. Accordingly, nearly any pointed object can serve as a stylus. Combination pen/stylus devices already exist for resistive touch screens. For example, the Dr. Grip 1+1 Stylus Pen Combo manufactured by PILOT® is a combination ballpoint pen and stylus for use with resistive touch screens. The tips of such devices are typically plastic or a similar polymer, so as not to damage or scratch the screen.
Capacitive touch screens are quickly replacing resistive touch screens, and are used with many modern small digital devices. For example, the newer iPhones® and iPads® from APPLE® are all equipped with capacitive touch screens. Capacitive touch screens generally comprise a flat insulative transparent sheet such as glass having an inside portion coated with a transparent conductor such as indium tin oxide (ITO), films made from graphene (carbon nanotubes), or other suitable material. Conductive materials that touch or are in very close proximity to this type of touch screen alter the electrostatic field of the screen, thereby creating a registerable change in capacitance. At the physical level a changing electrical potential difference causes a flow of electrons as an alternating current (AC) through a capacitor and it is this current flowing to a sink or source of electrons that is detected by the touch screen device. For some conductive materials such as biological tissue, these charged carriers could be predominantly ions—cations and/or anions. This sink or source of electrons, sometimes called a “ground” is necessary to complete the flow of electrons in most types of capacitive touch screens that can be activated by human touch. The effectiveness of a body as a ground is directly proportional to the product of its volume and conductivity. For alternating current and complex materials such as biological tissue it is also dependent on the frequency of the alternating current.
The most common input device used with capacitive touch screens is the human finger. Although the conductivity of the human body is not particularly high, its relatively high volume nevertheless allows it to act as an effective ground. At low frequencies typical biological tissues have conductivities on the order of 1 to 10 S/m (Siemens per meter) compared to metals like copper and aluminum which are 58 and 35 MS/m (million Siemens per meter) respectively. Traditional plastic or polymer-based styli are not effective in marking on capacitive touch screens because they are not sufficiently conductive. The problem is exacerbated if the user of the stylus is wearing gloves or has extremely dry skin. This is common in colder environments, where people may often need to mark on handheld devices while outside. These situations are problematic because the user is further insulated from the stylus which prevents the flow of alternating current to the human body ground. Though other materials providing better conductivity could be used, such as aluminum or other metals, they would likely scratch or otherwise damage the touch screen. Furthermore, many capacitive touch screens are tuned to detect inputs from human fingers and thus may not register a hard pointed input, simply due to the goal of minimizing false inputs.
One solution that enables a stylus to be used with a capacitive touch screen is the use of conductive rubber or a similar conductive elastomeric material. Conductive rubber is a rarer and more expensive form of rubber that contains suspended graphite carbon, carbon nanotubes, nickel or silver particles. Its electrical impedance decreases when it is compressed, and the capacitance increase as a result of the larger surface area in contact with the touch screen, thereby making it useful for capacitive touch screen applications. In addition, the rubber durometer can be set so as to deform at its tip in a manner similar to the deformation exhibited by a human finger as it presses down on a flat surface.
What is missing in the present art is a writing device that can seamlessly transition between marking on paper and marking on a capacitive screen. Such combinations for resistive-screen styli like the PILOT® pen proved easy because a rigid, non-conductive end of a pen could be used. For capacitive screens, no such device exists in the prior art because of the challenges with mounting a writing pen within a sufficiently flexible, sufficiently conductive material.